Time delay fuse

ABSTRACT

1. A time delay fuse comprising a consolidated body of noncombustible insulating fibers having enclosed therein a winding fuse path having two ends consisting of a portion of said noncombustible insulating fibers coated with a combustible composition which evolves substantially no gas on burning, said fuse path being defined and spaced from itself by said uncoated fibers, said ends of the fuse path being exposed on the surface of the consolidated body.

United States Patent [Ill 3,897,731

Bowman Aug. 5, 1975 1 TIME DELAY FUSE 2.796.834 6/1957 McCaffrey at al.7 102/27 2.923.239 2 I960 And -t 'l. t I02 27 [75} Inventor: Donald E.Bowman, Baltimore, Md. 3027 839 451962 gjy 102; [73] Assignee: CatalystResearch Corporation. FOREIGN PATENTS OR APPLICATIONS Bammore 746.8433/1956 United Kingdom 102/27 [22 Filed; 061. 30, 1961 619.533 5/1961Canada v. 102/27 [2]] Appl' NO; l49483 Primary ExaminerVerlin R.Pendegrass [52] US. Cl. 102/85; 102/27 R; 102/70 R EXEMPLARY CLAIM [5!]Int. Cl. C06C 5/00 53 Field of Search 102/27 85 85.2 85.6 A iconsol'duted g f 86 of non-combustible insulating fibers having enclosedtherein a winding fuse path having two ends consisting of a portion ofsaid non-combustible insulating fibers [56] References Cited coated witha combustible composition which evolves UNITED STATES PATENTSsubstantially no gas on burning, said fuse path being 2.440.579 /1948Fraz r l al 102/27 defined and spaced from itself by said uncoatcdfibers. 1493050 2/1950 selvitige 02/27 said ends of the fuse path beingexposed on the sur- 2,6l7.326 ll/l952 MOl'llS t a i 7 86/1 f of heconsolidated body 2.736.263 2/1956 Lewis et almn 102/27 1757.566 8/1956Towel] et al. 86/1 7 Claims, 3 Drawing Figures i 7 I8 22 I6 30 4 /0PATENTEU AUG 5l975 II 1 I l INVENTOR. DONAL 0 E. BOWMAN TIME DELAY FUSEThis invention relates to fuses of the type used with explosives, inpyrotechny and for related purposes, and more particularly to time delayfuses.

Time delay fuses now generally in use comprise a metal tube enclosing apyrotechnic material. Such fuses are often heavy and bulky, andfrequently do not provide precise and reproducible time delays.

It is an object of this invention to provide a reliable and accuratetime delay fuse. Another object is to provide a time delay fuse that islight and of small dimensions. A further object is to provide time delayfuses that can be simply and conveniently connected in series to provideany desired time delay. A still further object is to provide a timedelay fuse in which the pyrotechnic fuse path is enclosed within acompact mass of insulating fibrous material. Other objects will beapparent from the following description and claims.

According to this invention a fuse strip is formed by coating fibers ofa non-inflammable insulating material, such as glass fibers, ceramicfibers, asbestos fibers of mixtures thereof with a combustiblecomposition which evolves substantially no gases on burning, and formingthem into a cohesive matte or paper. The fuse strip is enclosed in anuncoated matte or paper of the fibrous material, and the entire assemblyis compressed into a dense compact mass. The resulting compact offibrous material thus contains a path of fibers coated with acombustible composition which is insulated from itself by the enclosinguncoated fibers. The fuse compact which is somewhat brittle, is enclosedby a container or coated with protective material and provided with anexposed ignition means and primer means connected to opposite ends ofthe fuse strip.

FIG. 1 is an exploded perspective view of a fuse element according tothis invention;

FIG. 2 is a sectional view of the compressed fuse element of FIG. 1enclosed in a housing; and

FIG. 3 is a perspective view of the fuse of FIG. 2.

The fuse strip of non-combustible insulating fibers coated with thecombustible composition may be formed in a variety of ways that will beapparent to those skilled in the art. A most convenient method is tomake a paper from the fibers using the combustible composition as afiller. That is, a slurry of the combustible composition is added andmixed with a slurry of the fiber and a filled paper is formed from theslurry mixture in a conventional manner using a Fourdinier machine,cylinder paper machine, or other conventional apparatus. The fuse strip,as well as other parts used to form the fuse compact, are maintained wetwith water, suitably about 7090 percent by weight water, throughout allprocessing steps until compacted to facilitate handling and forming andto prevent accidental ignition of the protechnic. If preferred, the fusestrip may be formed in layers of unfilled and combustible compositionfilled paper.

The fibrous material must be noncombustible and have a low heatconductivity such as glass fibers, asbestos fibers, or ceramic fibers.It is preferred to use fine, flexible fibers as the paper producedtherefrom is most flexible and more easily compressed to a dense compactthan papers formed from coarse fibers. In the following description theterm glass fiber may be used for brevity, it being understood that othernon-combustible fibers having a low heat conductivity are equallysuitable.

Combustible compositions that undergo exothermic reaction without theliberation of any substantial amount of gas are suitable for use in thefuse strips of this invention. In general, such compositions comprise anoxidizable substance and an oxidizing agent. with or without an inertdiluent to modify the rate of reaction. For most purposes, I prefercompositions comprising one or more finely divided metals having highheats of combustion as the oxidizable substance, and as the oxidizingagent any of a variety of inorganic substances that react therewith butwithout liberation of any substantial amount of gas, examples beingchlorates, perchlorates and nitrates, particularly of the alkali metals,as well as chromates, iron oxide, manganese dioxide and others. Thecomposition must, of course, be capable of ignition by such means as apercussion primer, match, or the well-known electric matches. Othercriteria are that the composition must be stable under normal storageconditions, and preferably have certain ignition at low ambienttemperatures. The particular combustible composition to be used willdepend upon the desired burning rate of the fuse. The characteristics ofthese compositions may in general be modified by adding or altering thecontent of an inert diluent, such, for example, as diatomaceous earthand similar incombustible and non-reactive materials. Many suchcompositions are wellknown in the art, and by way of illustration,reference may be made to the following compositions that have been foundto be satisfactory for the purposes of the invention.

The preferred combustible composition is composed of zirconium metalpowder (Zr) and not more than a stoichiometric amount of powdered bariumchromate (BaCrO This composition readily ignites at low temperatures andthe burning rate may be adjusted by varying the proportions of zirconiumand barium chromate. A suitable composition having a moderate burningrate contains l8/z% Zr and 8 l BaCrO A mixture of zirconium metal (Zr)powder (-200 mesh) and powdered red iron oxide (Fe O in stoichiometricproportions (43.5% Zr, 5l.5% Fe o with 5 per cent by weight ofdiatomaceous earth as a diluent exhibits fast propagation of combustion.

Another composition is composed of, by weight, 22 per cent nickel (Ni)powder, 5 per cent zirconium (Zr) powder, 16.8 per cent potassiumperchlorate (KCIO,) and 56.2 per cent barium chromate (BaCrO Thiscomposition ignites easily at low temperatures and the relativepercentages of the four components may be varied over a wide range togive various burning rates. The composition just stated after beingcompressed at 12,000 psi burns at the rate of 0.l inch per second, or aburning time of 10 seconds per inch. As exemplifying how the burningcharacteristics may be varied, if the percentage of zirconium be heldconstant and the nickel and potassium perchlorate be maintained insubstantially stoichiometric ratio, with the barium chromate beingvaried as a diluent, a burning time of 3 seconds per inch is to be hadwith 25 per cent of potassium perchlorate and 35.5 per cent of nickelpowder and barium chromate. On the other hand, at approximately 12 percent of potassium perchlorate, 15 per cent of nickel powder and 67 percent of barium chromate the burning time is increased to 18 seconds perinch.

When nickel is used in these compositions it is preferred that it beprepared by distillation of mercury (Hg) from a nickel-mercury amalgamas described and claimed in U.S. Pat. No. 1,893,879, issued Jan. 10,1933, on an application filed by Joseph C. W. Frazer et al. Such nickelpowder may be stabilized, if desired, as described and claimed in U.S.Pat. No. 2,487,632, issued Nov. 8, 1949, on an application filed by O.G. Bennett. This form of nickel powder is preferred because it has beenfound to be much more active in these combustible compositions thannickel produced in any other way. Another composition is one consistingof 9.6 per cent, by weight, of granular aluminum (Al) and the balancebarium chromate.

Referring now to FIG. I, which is an exploded view of the parts of thefuse compact 1, the fuse strip 2 is positioned on backing or insulatingstrip 4, an unfilled glass fiber paper. The fuse strip and backing stripare maintained wet with water and the fuse strip is conveniently rolledor calendered into the backing strip to hold it in position. A leadstrip 6 of combustible composition filled glass fiber paper ispositioned in contact with one end of the fuse strip 2 and extendsbeyond the backing strip. The fuse strip and backing strip are spirallywound around core 8 with the end adjacent to lead strip 6 on the insideof the spiral; the extended portion of the lead strip 6 is folded overthe upper face of the core. A second lead strip 10 is contacted with theouter end of the spirally wound fuse strip and a portion 22 extendsthrough slot 12 in insulating disc 14. A primer-disc 16 of glass fiberfilled with combustible composition is positioned in the opening 18 ofretaining ring 20 and is placed against the lower face of disc 14 incontact with the portion 22 of lead strip. An ignitable disc 24 alsoformed of glass fiber paper filled with combustible composition inretaining ring 26 is positioned to contact the portion of lead strip 6which overlies the upper face of core 8. All of the aforementioned partsof the fuse assembly are formed of compressable glass fiber paper; thefuse path, including ignitable disc 24 and primer disc I6, are of glassfiber paper filled with combustible composition.

During assembly, the parts are maintained wet with water, and while wet,the assembly is compressed to form a single consolidated body orcompact. The compression is accomplished in any conventional manner, thedie having weep holes for discharging water squeezed from the paper. Anycompression force may be used that compacts the element into a singleconsolidated body, e.g. 50-1000 psi. The burning rate of the fuse stripvaries considerably depending on the compression pressure; the higherthe compression pressure, the lower the burning rate. For example, anuncompressed glass paper strip filled with a composition of l8 /z% Zrand 8l/2% BaCrO, which burns at about 2 inches per second, burns at arate of l inch per second after compression under 750 psi.

The compressed fuse element is dried at any suitable temperature belowthe ignition temperature of the combustible composition, and is enclosedin a moisture proof container having openings to expose the ignitabledisc 24 and primer disc 16. Preferably, as shown in FIG. 2 and FIG. 3,cover discs 28 and 30 of laminated glassboard having openings 32 arepositioned on either end of the fuse compact. The glassboard covers areheld in place, and the sides of the fuse compact are protected by acoating of water impermeable resin plastic 34, such as an epoxide resin.The exposed ignitable disc and primer disc are then varnished and driedin the usual manner.

The operation of the time delay fuse is best described with reference toFIG. 2. When the ignitable disc 24 is ignited in any conventionalmanner, as by electrical match. it ignites lead strip 6 which in turnignites fuse strip 2. Fuse strip 2 bums linearly at a predetermined rateand in turn ignites lead strip 10 and primer disc 16. The primer disc inturn ignites a blasting cap or other means to set off a main explosivecharge. A number of fuse elements may be stacked, the primer disc 16 ofthe first element will then ignite the ignitable disc 24 of the nextfuse element, to provide any desired fuse delay time.

Preferably the ignitable disc 24 and lead strips 6 and I0 are filledwith a combustible composition that burns substantially instantaneously,e.g., 67% Zr and 33% Ba- CrO so that the only significant delay occursin fuse strip 2. Primer disc 16 is also filled with a substantiallyinstantaneous burning composition to provide sure ignition of a chargeor another fuse element.

It is apparent to those skilled in the art that fuse elements of anysize or shape other than that specifically described herein can be madeby the method of this invention, and that several fuse elements can becombined or stacked within a single container to permit assembly ofcompleted fuses with any desired time delay from a stock of only a fewsub-assembly fuse elements. Also, the fuses can readily be stacked in ajig or the like in the field to provide any desired time delay.

The following example is illustrative of this invention, wherein an 8.0second delay fuse is only 0.2 inch thick and l 7/16 inch in diameter.Fuse strip material was prepared by slurrying l5 g. of chopped superfineglass fiber having an average diameter of between about 0.2 and 0.499microns in 2% liters of water. About 4.5 g. of the fiber was laid downas paper on a 12 X l2 inch screen. Sixty-seven grams of BaCrO, and 15.2g. of Zr were added and mixed with the remaining glass fiber slurry andthe resulting slurry was laid down on the screen supported 12 X I2 inchglass fiber paper previously laid down to form a sheet of fuse material0.03 inches thick. The use of an unfilled backing layer is preferred toprovide a more flexible fuse material that can be wrapped in very smalldiameters without breaking. Lead strip material .03 inches thick wassimilarly prepared, except a 67% Zr 33% BaCrO, filler was used.

The ignitable disc and primer disc were formed from a 0.4 inch thickpaper containing 25% superfine glass fiber, 37%70 ceramic fiber having acomposition of approximately 50% alumina and 50% silica and having anaverage diameter of about 4 microns, and 37Vz% fine, flexible asbestosfiber, filled with a combustible composition of 67% Zr and 33% BaCrO Nobacking layer is used on the ignitable disc or primer disc material,since these parts must be homogeneous so that they burn on both sides.

The following parts were cut from the appropriate combustiblecomposition filled stock and from superfine glass fiber paper stock ofappropriate thickness and assembled as heretofore described: (l a glassfiber paper backing strip 4, 8.0 inches long, 10/32 inches wide, 0.08inches thick; (2) a fuse strip 2, 8.0 inches long, 5/32 inches wide and0.03 inches thick; (3) a glass fiber core 8, inches in diameter and 0.3inches thick; (4) a lead strip 6, 1 inch long, V4 inch wide and 0.03inch thick and a lead strip 10, 2 inches long, V4 inch wide, and 0.03inch thick; (5) a glass fiber paper insulating disc, 14, 1 3/16 inchesin diameter and 0.05

inch thick; (6) glass fiber paper annular retaining rings and 26 with aninside diameter of k inch, an outside diameter of 1 3/16 inches and athickness of 0.100 inch; and (7) an ignitable disc 24 and primer disc16, V2 inch in diameter and 0.4 inches thick. The assembly wascompressed at 750 psi and dried to form a compact fuse element 1 3/16inches in diameter and .15 inch thick. Heat resistant, laminatedglassboard annular covers 28 and 30, l 7/16 inches in diameter and 0.025inch thick were placed on both faces of the fuse element, coated withepoxide and varnished as heretofore described.

According to the patent statutes, 1 have explained the principle of myinvention and have illustrated and described what I now consider torepresent its best embodiment. However, I desire to have it understoodthat, within the scope of the appended claims, the invention may bepracticed otherwise than as specifically described.

I claim:

1. A time delay fuse comprising a consolidated body of noncombustibleinsulating fibers having enclosed therein a winding fuse path having twoends consisting of a portion of said non-combustible insulating fiberscoated with a combustible composition which evolves substantially no gason burning, said fuse path being defined and spaced from itself by saiduncoated fibers, said ends of the fuse path being exposed on the surfaceof the consolidated body.

2. A time delay fuse according to claim 1 in which the fibers are glassfibers.

3. A time delay fuse comprising a cylindrical consolidated body ofnon-combustible insulating fibers having therein: l an enclosed spiralfuse path on a plane normal to the axis of said cylindrical body andconsisting of a portion of said non-combustible insulating fibers coatedwith a combustible composition which evolves substantially no gas onburning, (2) ignitable means in said cylindrical body having an exposedsurface and connected to one end of said fuse path, and (3) primer meansin said cylindrical body having an exposed surface and connected to theopposite end of said fuse strip.

4. A time delay fuse according to claim 3 in which the fibers are glassfibers.

5. A time delay fuse comprising a consolidated body of noncombustibleinsulating fibers having therein a first ignitable disc having anexposed surface consisting of a portion of said fibers coated withcombustible material which evolves substantially no gas on burning, asecond ignitable disc having an exposed surface and spaced from saidfirst ignitable disc consisting of a portion of said fibers coated witha combustible material which evolves substantially no gas on burning,and an enclosed winding fuse path consisting of a portion of said fiberscoated with a combustible composition which evolves substantially no gason burning, said fuse path being defined and spaced from itself by saiduncoated fibers, one end of said fuse path contacting said first discand the opposite end of said fuse path contacting said second disc.

6. A time delay fuse according to claim 5 in which the ignitable discsare fast burning and the fuse path is slow burning.

7. A time delay fuse comprising a cylindrical consolidated body ofnon-combustible fibers having top and bottom surfaces and having thereina combustible path consisting ofa portion of said fibers coated with acombustible material that evolves substantially no gas on burning, saidcombustible path consisting of l) a first ignitable disc normal to andon the axis of said body and having an exposed surface on said top face;(2) a second ignitable disc spaced from said first disc and having anexposed surface on the said bottom face, said second disc being normalto and on the axis of said body; (3) an enclosed fuse path comprising aspiral on a plane normal to the axis of said body and between saiddiscs, one end of said fuse path contacting said first disc and theother end of said fuse path contacting said second disc.

1. A time delay fuse comprising a consolidated body of noncombustibleinsulating fibers having enclosed therein a winding fuse path having twoends consisting of a portion of said non-combustible insulating fiberscoated with a combustible composition which evolves substantially no gason burning, said fuse path being defined and spaced from itself by saiduncoated fibers, said ends of the fuse path being exposed on the surfaceof the consolidated body.
 2. A time delay fuse according to claim 1 inwhich the fibers are glass fibers.
 3. A time delay fuse comprising acylindrical consolidated body of non-combustible insulating fibershaving therein: (1) an enclosed spiral fuse path on a plane normal tothe axis of said cylindrical body and consisting of a portion of saidnon-combustible insulating fibers coated with a combustible compositionwhich evolves substantially no gas on burning, (2) ignitable means insaid cylindrical body having an exposed surface and connected to one endof said fuse path, and (3) primer means in said cylindrical body havingan exposed surface and connected to the opposite end of said fuse strip.4. A time delay fuse according to claim 3 in which the fibers are glassfibers.
 5. A time delay fuse comprising a consolidated body ofnoncombustible insulating fibers having therein a first ignitable dischaving an exposed surface consisting of a portion of said fibers coatedwith combustible material which evolves substantially no gas on burning,a second ignitable disc having an exposed surface and spaced from saidfirst ignitable disc consisting of a portion of said fibers coated witha combustible material which evolves substantially no gas on burning,and an enclosed winding fuse path consisting of a portion of said fiberscoated with a combustible composition which evolves substantially no gason burning, said fuse path being defined and spaced from itself by saiduncoated fibers, one end of said fuse path contacting said first discand the opposite end of said fuse path contacting said second disc.
 6. Atime delay fuse according to claim 5 in which the ignitable discs arefast burning and the fuse path is slow burning.
 7. A time delay fusecomprising a cylindrical consolidated body of non-combustible fibershaving top and bottom surfaces and having therein a combustible pathconsisting of a portion of said fibers coated with a combustiblematerial that evolves substantially no gas on burning, said combustiblepath consisting of (1) a first ignitable disc normal to and on the axisof said body and having an exposed surface on said top face; (2) asecond ignitable disc spaced from said first disc and having an exposedsurface on the said bottom face, said second disc being normal to and onthe axis of said body; (3) an enclosed fuse path comprising a spiral ona plane normal to the axis of said body and between said discs, one endof said fuse path contacting said first disc and the other end of saidfuse path contacting said second disc.